With summer in full swing in the Northeast and both temperature and humidity rising quickly, many athletes notice a significant change in their body’s response to exercise.

Whether it is running or cycling on the first warm day of spring or summer or training in a more humid environment while away on vacation, we all notice how our bodies, and training potential, are impacted by different environmental factors. These environmental and modifiable risk factors contribute to the amount of heat stress placed on an athlete's systems. And while we can’t always change the weather that we train or compete in, we may be able to decrease exercise intensity and modify our training gear to avoid heat injuries and illness.

Heat acclimatization is the body’s way of acclimating and adjusting it’s response to exercise in the presence of increased humidity and heat stress during physical activity. By exercising in warmer, more humid conditions the body’s core temperature increases quicker, as compared to exercising in cooler or drier conditions. Therefore, in order to maintain homeostasis and avoid heat injuries the body increases its sweat response as a way to keep the body cool.

What is heat acclimatization or heat acclimation?

Heat acclimatization is the body’s ability to physiologically adapt to heat stress to avoid heat injuries. When exercising in warmer conditions than what the body is accustomed to, we see an increase in body temperature, cardiovascular strain, impaired aerobic performance and increased risk of heat illness. The three main signs of adequate heat acclimatization are:

  1. Lower heart rate response to physical activity
  2. Lower core body temperature
  3. Higher sweat rate during exercise with heat stress

Humidity then further strains the body’s systems by inhibiting the amount of evaporation of sweat that can take place, which aids cooling the body. If the body is unable to adapt to such changes in environment, athletes risk suffering from heat injuries and illness, such as heat exhaustion, heat stroke and dehydration. These injuries and illnesses pose a threat to our athlete’s wellbeing and ability to safely participate in their sport to their full extent. Therefore, acclimating and safely adjusting to an increase in heat and humidity is of the utmost importance when transitioning to outdoor training in the spring and summer months.

What to expect during heat acclimatization training?

Different systems in the body react and adapt to new environments at different paces. Heat acclimatization can take 7-14 days for an athlete to see their full benefits. However, some physiological changes can be seen as early as 3-5 days into heat acclimatization training. Aerobically trained athletes will adjust up to 50% quicker to new environments and heat acclimatization may be maintained for longer periods of time.

  • Heart rate acclimates the quickest, with a decrease in heart rate response during exercise initially seen within 3-5 days. After 7 days athletes gain the most benefit from this.
  • Thermoregulatory benefits, or the body’s ability to maintain a safe internal temperature, are seen after 10-14 days for most athletes. This includes but is not limited to a decreased sweat response and improved regulation of core body temperature. Some studies have shown that small benefits can be gained after 14 days.

Most heat acclimatization protocols recommend heat exposure for 90 minutes each day, during which point your cardiovascular training should take place. This can be broken into two shorter segments of 60 minutes each. Athletes should take the time to ease into training and pay close attention to how their body reacts to these environmental factors.

The effects of training in the heat are maintained for about 1 week if training in hot and humid conditions is not sustained. After this time the adaptations made by your body decay by 75% within three weeks. In order to maintain the benefits, athletes should intersperse training in their hot and/or humid environment every 2 to 3 days.

How can I prepare to exercise in the heat safely?

The most important part of starting any new exercise routine or new environment is to check in with your body and how you react. Be sure to set yourself up for success.

  • Adequately hydrate 2-3 hours before exercising in the heat.
  • Wear loose fitting, lightweight clothing that allows for sweat to evaporate off the skin and continue to aid in cooling the body.
  • When you are beginning your heat acclimatization training, begin by training in the early hours of the day when the temperature and humidity is typically lower.
  • Decrease the pace at which you run or cycle in order to allow for your body to adjust and the physical strain to be lower during physical exercise.
  • Be sure to hydrate while exercising, prior to feeling thirsty.
  • After exercising, be sure to adequately rehydrate with water.
    • Sports beverages typically are not needed unless exercising for prolonged periods of time (>1-2 hours)
  • Do not skip meals, as this is when your body replaces most of the fluid and sodium lost during physical activity
  • During your first week of training in the heat and humidity, be sure to pay extra attention to your fluid and food intake as this is when your sweat production will be greatest

Am I at an increased risk of developing heat illness, injury or dehydration?

Women and older adults (>65 years old) are also at an increased risk for fluid and electrolyte imbalances during and after exercise. Older adults have a suppressed thirst response, which contributes to this increased risk. This is one of the reasons it is important to begin walking prior to feeling thirsty. Women have lower sweating rates and electrolyte losses, as compared to men, due to smaller body size and lower metabolic rates when exercising. However, women have a greater diuretic response to water load than men, which can lead to women turning water over quicker, potentially leading to dehydration during exercise if not managed and monitored appropriately.